Speciation and bioavailability of heavy metals in paddy soil irrigated by acid mine drainage

Based on field investigation and laboratory analysis,the speciation and bioavailability of the heavy metals(Cd,Cu,Pb,and Zn) was studied.These metals were tested from 16 soil samples taken from paddy soils irrigated by acid mine drainage in the lower stream of Dabaoshan mine area,Guangdong Province,China.The toxicity characteristic leaching procedure(TCLP) and three-step community bureau of reference(BCR) sequential extraction procedure methods were used to analyse the bioavailability of heavy metals.TCLP is currently recognized as an international method for evaluation of heavy metal pollution in soils.Two methods were compared in bioavailability by stepwise multiple linear regression analysis.Cd was dominated mainly by the acid-extractable and residual fractions,and followed by the order of acid-extractable residual oxidisable reducible.Cu was dominated mainly by the residual fraction,and decreased in the following order:residual reducible acid-extractable oxidisable.Pb was dominantly released in the reducible and residual fractions,they account for 86.54% of the total contents,and decreased in the following order:reducible residual acid-extractable oxidisable.Zn was dominated mainly by the residual fraction,it takes more than 50% of the total contents,and followed by the order of residual acid-extractable reducible oxidisable.The levels of Cd,Cu,Pb and Zn extracted by TCLP were 0.17-0.89,8.12-70.33,3.16-90.33 and 10.24-106.85(mg·kg~(-1)),respectively,and the average concentrations of Cd,Cu,Pb and Zn were 0.41,36.60,15.97 and 50.78(mg·kg~(-1)),respectively.The concentrations of Cd,Cu,Pb and Zn in rice seeds were 0.183-0.947,3.542-5.997,0.285-1.532 and 17.54-41.10(mg·kg~(-1)),respectively.Comparing to the national standards on heavy metals in foods by the Chinese Ministry of Health,the contents of Cu and Zn in rice seeds didn't exceed the upper limit,while the contents of Cd and Pb in rice seeds exceed the upper limit by 87.50% and 81.25%,respectively.Results of the stepwise multiple linear regression analysis revealed that contents of Cd and Zn in seeds of rice were significantly influenced by contents of acid-extractable Cd and Zn,that contents of Cu in seeds of rice were significantly influenced by contents of acid-extractable and oxidisable Cu,and the contents of Pb in seeds of rice were significantly influenced by contents of reducible Pb and organic matter.Contents of four metals in seeds of rice were significantly influenced by contents of metals extracted by TCLP,and the influences of total content,pH and organic matter were not significant.In short,TCLP method was fast,simple and suitable for evaluation of bioavailability of heavy metals in sewage-irrigated paddy soils by acid mine drainage.